In recent years, with the advance of environmental technology, development of power generation devices (e.g., solar power generation devices) which pose less burden on the environment than power generation devices using conventional power generation methods has been actively conducted. Concurrently with the development of power generation technology, development of power storage devices has also been underway.
As an example of a power storage device, a lithium ion battery which is a secondary battery can be given. A lithium ion battery is widely prevalent because energy density thereof is high and the lithium ion battery is well suited for miniaturization. As an electrode material of a lithium ion battery, a material in/from which lithium ions can be inserted and extracted is preferably used, and graphite, silicon, and the like can be given as examples of the electrode material of a lithium ion battery. In particular, the theoretical capacity of silicon is about ten times as high as that of graphite; thus, silicon is expected to be a host material of lithium.
However, there has been a problem in that, when LiPF6, LiBF4, LiAsF6, or the like is used as an electrolyte in a secondary battery including an electrode containing silicon or a silicon compound, sufficient charge-discharge cycle characteristics and sufficient storage characteristics are not obtained. Patent Document 1 discloses a secondary battery in which concentration of at least one of these electrolytes is less than 0.1 mol·dm−3, whereby sufficient charge-discharge cycle characteristics and sufficient storage characteristics are obtained.